In addition to the main fuel, energy-conversion devices, or converters, such as internal combustion engines, also use auxiliary fuels during operation, for example, oil for lubrication and water for cooling. The use of lubricating oil and cooling water, however, is not tied directly to the fuel consumption and is generally very low. As a result, triggering a warning indicator is sufficient for refilling the fuel supply, depending on the time and, with vehicles, also depending on the distance.
Modern internal combustion engines, in particular, those used in motor vehicles, have auxiliary units that require a consumable auxiliary fuel supply for operation, wherein the operation of the auxiliary units is tied directly to the operation of the internal combustion engine. Auxiliary units of this type include, for example, exhaust-gas treatment systems, which are used to reduce nitrogen oxide emissions and to which additives for the exhaust-gas after treatment must be added, e.g. so-called selective catalytic reduction (SCR) catalysts supplemented with reducers. The auxiliary fuel amount to be supplied depends on the exhaust gas amount to be treated and, thus, for all practical purposes directly on the amount of the main fuel that is consumed.
A fuel cell, for example, can be considered to be an auxiliary unit for the autonomous energy supply for electric auxiliary units, wherein the fuel cell is supplied by an inserted hydrogen cartridge as fuel. The main fuel is, thus, used only for the operation of the internal combustion engine, while at least a portion of the electrical energy required for operating the auxiliary units can be generated by the autonomous electrical energy supply without stressing the environment. Such auxiliary units, operated via an autonomous electrical energy supply, advantageously include all auxiliary units that are not directly connected to the operation of the internal combustion engine, such as air-conditioning systems, window raising/lowering devices, communication means, and servo-units, e.g. for supporting the steering or the braking functions.
The respective remaining main and auxiliary fuel supplies, at least the reaching of a minimum fuel level, have so far been detected with sensors, and a corresponding warning signal generated.
For example, if the internal combustion engine comprises an exhaust-gas after-treatment device which requires a consumable fuel supply for an orderly operation, e.g. feeding a reduction means for a SCR catalyst, the internal combustion engine can still operate even if the fuel for the exhaust-gas after-treatment device is used up. The disadvantage, however, is that the operation then generates an impermissibly high amount of nitrogen oxide.
A fuel cell represents another application of an energy-conversion device, or converter. In the fuel cell, electrical current is generated through the electro-chemical conversion of a hydrogen-containing combustion gas as the main operating fuel in a reaction with another gas, such as air. An auxiliary fuel such as water, for example, is used for wetting the reaction gas and is needed to adjust a defined water content in the reaction air.
If the fuel cell functions as a main drive or as an auxiliary drive of a vehicle, a main fuel and an auxiliary fuel must respectively be carried along in separate reservoirs, in the same way as for the internal combustion engine.
Energy-conversion devices are henceforth simply referred to as converters, including the combustion engine mentioned as an exemplary embodiment for an energy-conversion device, and the fuel cell.